There has always been a trade off between traction and maneuverability in vehicles, particularly heavy equipment vehicles, tractors, and the like. Tractors were generally not considered for four wheel drive systems because it was believed that deliberate weight distribution and bilaterally independent brakes on the drive wheels would be sufficient for maximum tractive force. Moreover, there are difficulties in providing a driving force to the front steering wheels of such vehicles. If the steering wheels were given drive capability, it would be advantageous to make the steering wheels larger for greater traction. However, larger front steering wheels would interfere with the framework of the vehicle, thereby limiting their usefulness. In addition, conventional differentials required to drive the steering wheels are complex and expensive. With the advent of zero turning radius vehicles capable of turning in a reverse direction within the length of the vehicle, there is a greater need for improved traction. Some such vehicles have steering wheels which pivot about vertical axes in place of less directionally stable caster wheels. The present invention relates to zero turning radius vehicles in which the steering wheels are both driven and rotated about their vertical axes to control the steering and propulsion forces thereto.